Bombardier will have to wait an extra two months to find out if its CSeries commercial jets will be hit by punishing U.S. duties.

The U.S. Commerce Department on Wednesday granted Boeing's request to delay its preliminary determination on the aircraft giant's petition until Sept. 25 from July 21.

A final determination is still expected later this year.

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In April, Boeing petitioned the U.S. Commerce Department and U.S. International Trade Commission, alleging that the Montreal company has been selling CSeries planes in the U.S. below cost thanks to public subsidies in violation of trade rules.

Bombardier (TSX:BBD.B) has rejected the allegations and argued that their CSeries planes never competed with Boeing in a key sale to Delta Airlines.

Boeing said respondents such as Bombardier have been granted several extensions and that the complexity of the financial support provided to Bombardier requires that the investigation take more time.

"It is important that the department have sufficient time to investigate each of these subsidies thoroughly," Boeing wrote in its letter.

Boeing has asked the U.S. government to impose preliminary countervailing duties of 79.41 per cent, followed later by anti-dumping duties of 79.82 per cent.

"Bombardier agrees that the Department of Commerce should have sufficient time to examine the evidence that demonstrates that Boeing's motion is unfounded," Bombardier spokesman Bryan Tucker said.

Boeing declined to comment on its request, which a spokesman described as "routine" in complex cases.

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The Bombardier-Boeing dogfight has also taken on a political dimension. The Canadian government said it is reviewing bids from the Chicago-based company for military contracts, a hint that Ottawa may be reconsidering its purchase of Boeing's Super Hornets.